Ionization radiation detector system for determining position of the radiation

ABSTRACT

An X- and gamma -radiation detector including a plurality of detecting cells. Each cell comprises an anode and two halfcathodes, the half-cathodes consisting of flat plates. The respective half-cathodes are arranged as othogonal elements so that by applying matrix technique the output of a particular cell can be selected to localize the radiation detected.

United States Patent Allemand et al.

[ 1 Nov.21, 1972 [S4] IONIZATION RADIATION DETECTOR SYSTEM FORDETERMINING POSITION OF THE RADIATION [72] Inventors: Robert Allemand,Grenoble; Christian Brey, Sassenage; Jean Jacobe, Montbonnot, all ofFrance [73] Assignee: Commissariat A LEnergie Atomique, Paris, France[22] Filed: May 22, I970 21 Appl. No.: 39,763

[30] Foreign Application Priority Data May 23, 1969 France ..6917042[52] US. Cl. .....250/83.6 R, 250/71.5 S, 250/83.3 R, 315/169 R [51]Int. Cl ..G01t 1/18 [58] Field of Search .....250/83.6 R, 83.3 R, 83.3HP, 250/71.5 S, 71 R; 315/169 R [56] References Cited UNITED STATESPATENTS 3,359,421 12/1967 Perez-Mendez et a1 ..25()/83.6R 2,877,3713/1959 Orthuber et a1 250/71 R X Primary Examiner-James W. LawrenceAssistant Examiner-Morton J. Frome Attorney-Cameron, Kerkam & Sutton[57] ABSTRACT V An X- and 'y-radiation detector including a plurality ofdetecting cells. Each cell comprises an anode and two half-cathodes, thehalf-cathodes consisting of flat plates. The respective half-cathodesare arranged as othogonal elements so that by applying matrix techniquethe output of a particular cell can be selected to localize theradiation detected.

8 Claims, 4 Drawing Figures IONIZATION RADIATION DETECTOR SYSTEM FORDETERMINING POSITION OF THE RADIATION The invention relates to aradiation detector system suitable for detection of X,'y, or Bradiation.

One application of the system is to the angular localization andmeasurement of the intensity of radiation diffracted by a small samplefor analysis irradiated by an X-ray beam.,The conventional method, andin practice the only method hitherto used for analyzing the spatialdistribution of the X or 'y electromagnetic radiation of a small sampleconsists in sweeping space by a suitable detector disposed on the arm ofa goniometer which can move angularly around the sample. Since theamount of charge corresponding to each particle to be detected is toolow to be directly measured by a preamplifier (below a threshold of theorder of 4 X coulombs) a proportional detector must be used, so as totake advantage of the coefficient of multiplication of the chargesprovided by this method of operation. The sweep method takes a long timeand has many disadvantages, since measurements are not madesimultaneously for all orientations.

There is a similar problem in X or y mapping of a large sample (such asan organ). Hitherto a scanning method has been used in which the spacein front of the sample is scanned by a detector having a collimationsystem appropriate to the kind of radiation. This method also has theaforementioned disadvantages, one of which is to take too long.

It is an object of the invention to provide a stationary detector systemfor electromagnetic and B radiations, which supplies informationsimultaneously for the whole of the space investigated.

To this end the invention provides a system which comprises, in a singleenclosure of constant thickness occupied by a fluid, a plurality ofidentical cells disposed at regular intervals, each comprising afilamentary anode and two fiat half-cathodes, and means for taking fromthe two half-cathodes, during the detection of a radiation, two electricpulses which are applied to a localizing device which provides aone-to-one correspondence between each cell and the association of twochannels for outputting said pulses, a dc. potential difference suchthat the cell operates proportionally being applied between the anodeand the two half-cathodes of each cell.

Preferably the two half-cathodes have substantially the same area andare disposed at the same distance from the filamentary anode, so thatidentical means for taking electrical pulses can be used for eachhalfcathode.

The localising device may comprise a matrix network, the two electricalpulses from a cell being applied, one to a line, the other to a columnof the network, and an addressing device for a pulse resulting at apoint of the network.

Since localizing the signals are taken from the halfcathodes, the halfcathodes may be at the DC. potential of earth, thus greatly simplifyingthe problems of signal output, and the filamentary anodes are at thatpositive potential in relation to earth which is required for operationas a proportional counter.

In an embodiment of the invention the anodes are formed by one or morelayers of wires, the wires of one layer all being parallel with oneanother, and the halfcathodes of the same cell are associated, one witha first group formed by parallel strips disposed in an identical plane,the other being associated with a second group also formed by parallelstrips disposed in a plane parallel with the first plane, the strips ofthe first and second groups being disposed on either side of the layerof anode wires and at an equal distance from such layer.

In a first embodiment of the invention, the parallel strips of the firstand second groups are parallel with the same direction, while in asecond embodiment of the invention they are parallel with twoperpendicular directions respectively.

The first arrangement disclosed hereinbefore is used in cases in whichsweeping in one plane is required (the meridian plane in studying thediffraction spectrum of X or 7 radiation).

In most cases the signals taken from the halfcathodes must, be subjectedto amplitude selection therefrom and processed to avoid errors caused bythe action of the same event on a number of adjacent cells. Those errorsmust also be eliminated which are caused by the appearance on two cellsof signals separated by an interval of time less than the resolutiontime of the input circuits of the counters associated with the cells.

To this end inhibition circuits can advantageously be provided toprevent the recording of two events separated by an interval of timeless than the recovery time of such circuits.

The invention will be more clearly understood from the followingdescription of nonlimitative exemplary embodiments thereof, withreference to the accompanying drawings, wherein:

FIG. 1 is a diagrammatic perspective view of a detector adapted toazimuthal and elevational localization of the distribution of an X or 7radiation coming from a substantially point source;

FIG. 2 is a diagram showing a fraction of the network of coincidencecircuits and the matrix localizing network which are associated with thedetector illustrated in FIG. 1;

FIG. 3, which is similar to FIG. 1, shows a detector for exploration ofa beam along a meridian plane of symmetry which can, for instance, be anazimuthal plane; and

FIG. 4 is a detail showing the two half-cathodes of a cell associatedwith a detector according to a modified embodiment of the invention.

The system illustrated in FIGS. 1 and 2 is adapted to determine theazimuthal and elevational angular distribution of the intensity of X or'y/radiation.

It can also be used for X,'y or B mapping of a sample whose dimensionsare of the same order as the area of the detector. In that use acollimator, formed, for instance, by a plate of heavy material formedwith suitably orientated orifices, is interposed between the detectorand the sample. Collimators of this kind are currently used for 'y or Xmapping by means of static detectors such as spark chambers orscintillation scanning cameras.

Before giving a full description of the system according to theinvention, it may be advantageous to recall a few facts aboutproportionally operating ionization detectors and the consequencesderivedtherefrom by inventors.

It is known that when the amount of charge delivered by a particle to bedetected in an ionisation chamber is too low to be directly measured bya preamplifier i.e., lower than a value of the order of 4 X coulombs thedetector must operate proportionally to take advantage of thecoefficient of multiplication of the charges due to the intense electricfield around the anode: the result is highly efficient detection. On theother hand, hitherto this method of operation has meant that underproportional conditions the counter tubes must be of preciselydetermined geometry; the counter tubes as a rule take the form of ametallic cylinder a few centimeters in diameter which forms the cathode,and a wire a few tens of p. in diameter disposed along the axis of thecylinder and forming the anode of the counter tube. The multiplicationzone for the electric charges due to sensing events is limited to a thincylindrical volume (a few tens of p. in thickness) around the anodewire, namely the space in which the electric field is adequate for theprimary electrons created by the radiation to acquire enough energybetween two collisions to ionize fresh gas molecules occupying thedetector enclosure. Whatever the place in the detector where the initialionization occurs may be, everything takes place as if the charges werecreated in full in the vicinity of the anode wire. The amount of chargesper unit of area (density of charges) produced by influence on thecylinder forming the cathode is therefore substantially constant for anygiven cross-section of the cylinder, although the amount varies alongthe given cross-section of the cylinder, although such amount variesalong the cylinder and has its maximum at the level of the cross sectionwhere the multiplication of the charges around the anode took place.Consequently, if the cylinder forming the cathode of a convcntionalproportional counter tube is divided into two half-cylinders, the samequantity of charges will be created by influence on each half-cylinder.

In these conditions, the inventors saw the possibility of drawing fromtwo half-cathodes the two items of information required to localize anyevent due to a radiation. Preferably, the two half-cathodes have thesame area and are so arranged in relation to the anode that the chargescreated by influence on the two halfcathodes are substantially identicali.e., the treatment networks for the signals collected can be identicalalthough such features are clearly non-limitative.

Due to this arrangement, the anode at high positive voltage can actsimply as an amplifier, and all the data required can be drawn from thehalf-cathodes at DC. earth potential, thus greatly simplifying thetechnology of the detector. More particularly, there is no need toprovide guard rings, and the seal between the information outputs andthe enclosure no longer have to stand up to the DC. operational highvoltage of the system, and the preamplifiers require no very highvoltage filters.

Referring to FIG. 1, a detector comprises a shallow air tight enclosure10 made of a low-absorption material. In the embodiment illustrated, theenclosure 10 (shown in chain-dot lines) is flat. As a matter of fact,the best theoretical shape for the detector to investigate radiationfrom one point is part-spherical, but as a rule such an embodiment istoo complicated to be justifiable. However, the enclosure can easily becurved into a portion ofa cylinder.

The enclosure 10, whose thickness can vary from a few millimeters to afew centimeters, is occupied by an atmosphere selected in dependence onthe energy of the radiation to be detected. As a rule, xenon, krypton,argon or a mixture thereof will be used for X radiation.

The enclosure 10 is occupied by a number N X n of detection cells, nbeing advantageously equal to N. Each of the cells comprises a firsthalf-cathode borne by an insulating member 12 covering one of the majorsurface walls (the wall opposite the source) or forming such wall, and asecond half-cathode borne by an insulating member 14 covering or formingthe opposite surface. In the embodiment illustrated in FIG. 1, the firsthalf-cathodes of N cells disposed in the same column are formed by asame metal strip disposed on the insulating support 12. There areprovided n identical metal strips 16 16 16 parallel with one another andseparated by equal intervals. Similarly, the second halfcathodes of ncells grouped on the same line are formed by a metallic strip on theopposite insulating support, and there are N such strips 18,, 18 18disposed perpendicularly to the strips 16 16 16,. Advantageously, themetal strips associated with the two groups have the same width and aredisposed at the same distance apart.

The anodes are disposed in the median plane between the cathode strips.

In the embodiment illustrated in FIG. 1, the anodes are formed by alayer of wires 19 19 19,, parallel with the strips 16 16 l6, anddisposed in the median plane thereof. Two superimposed planes of wiresforming a grid could be used, each plane being formed by wires eachdisposed in the median plane of one of the strips of correspondinggroup.

A DC. high voltage selected for the cells to operate proportionally isapplied to each of the wires forming the anodes via a common resistor22; similarly, each of the strips 16 16 forming the first half-cathodesis earthed via a resistor 24,, 24 and each of the strips 18,, 18 isearthed via a resistor 25,, 25

In a system used as an X-ray detector having an energy higher than 20KeV the strips of half-cathodes were formed by resilient printed circuitsheets (metal coating of 15p. in thickness on p. oftetrafluoroethylene). The distance between the two planes ofhalf-cathodes was 3 cm, and the pitch between the cells was 8 mm. Thediameter of the anode wires was 30p and the d.c. high voltage was about4,000 volts for an argon atmosphere at 5 bars. The out put surface wasof sintered beryllium. The surface through which the X-rays enter theenclosure was made of mylar and supported by a collimator with parallelapertures.

To detect a hard {3 radiation, it is preferable to use a mixture ofargon and carbon dioxide at a few bars, and a high voltage of the orderof 4,000 volts for the conditions set forth hereinbefore.

N X n counting cells, each corresponding to the coincidence between theinformation received on one line and one matrix column are associatedwith the detector illustrated in FIG. 1 via N n measuring channels(grouped in a matrix with n lines and N columns).

An address system 26 enables this correspondence to be carried out. Thenumbers of order 1 in a line and c in the column are coded, forinstance, in a coded decimal binary system, and by combining l and c theaddress can be obtained, for instance, y 1 k c, to which a pulsecorresponding to the coincidence line 1, column 0 is sent. is apredetermined integer of sufficient value to prevent any ambiguity,preferably a whole power of As stated hereinbefore, false informationmust be eliminated which is due either to the action of the same nuclearevent on a number of cells, or to the succession of two events on twodifferent cells in a very short interval of time less than theresolution time of the input coincidence circuits of the counters; theconsequence of the latter would be to record two events which did notreally take place, in addition to two real events, the system beingincapable of distinguishing that one of the four points disposed in arectangle adjacent which the nuclear event took place.

To eliminate the first cause of error (the same nuclear event as a ruleinfluences two or three cells), use can be made of the fact that thecharges collected on the half-cathodes are the charges produced by theelectrical influence of the displacement of the electrons in the gas,and that the quantity of charges collected therefore depends on theangle at which the halfcathodes are subjected to the displacement ofelectrons caused by the event. As a consequence, the erroneousinformation can be eliminated I by amplitude discrimination. I

To this end the circuit illustrated in FIG. 2 comprises amplitudediscriminators and inhibition circuits adapted to eliminate the secondcause of errors. To simplify the drawing, FIG. 2 shows only theinhibition circuits associated with four bands 16, 16

A pulse from the strip 16, is amplified by a preamplifier 29 and appliedto an amplitude discriminator 30 whose threshold is selected independence on the energy of the radiation to be detected. The output ofthe discriminator 30 is applied to one of the inputs of an AND circuit32 whose output controls a'monostable 34,. The output of that monostableis applied to the address circuit 26 on one of the scalers or countingrate meters associated with the strip 16 The outputs of all the ANDcircuits 32 32 are also connected to a NOR circuit 36 which controls thechange of state of the monostable 38 whose output is applied to thesecond input of each of the AND circuits 32 32 32 Clearly, therefore,when a pulse appears on one of the strips 16 16 the output of thecorresponding AND circuit 32,, 32 actuates via the NOR circuit 36 themonostable 38 which cuts off all the AND circuits 32 during a suitableinterval of time (for instance,

2 microseconds). This cutting off of the transfer channels to all thecounters 28 28 by the address circuit 26 after the appearance of a pulseon one of such channels takes place after about nanoseconds whenconventional integrated circuits are used: the risk of wrong informationis therefore limited to the very slight probability that two channelswill receive two pieces of information during an interval of time lessthan 20 nanoseconds. The monostables 34,, 34 34 enable a pulse ofsufficient length to be maintained on the address circuit 26 to assurethe routing and recording of the pulse by the corresponding counter 28 Asecond NOR circuit (not shown) performs the same function as the circuit36 for the strips l8,,l8 l8 The detector illustrated in FIG. 1 enablesthe distribution of beams over an area to be explored: when it isacceptable to determine distribution along a meridian, the arrangementshown in FIG. 3 can be adopted. In FIG. 3 members corresponding to thoseshown in FIG. 1 have like reference numbers followed by the primesymbol.

The system shown in FIG. 3 comprises apart cylindrical enclosure 10'adapted to be placed with its central plane coinciding with theazimuthal plane to be investigated. The opposite walls of the cylinderare covered with insulating layers 12', 14' bearing the halfcathodes l6'16 16,, and the half-cathodes 18',, 18' 18' respectively. The anodes19',, l9 19,, are formed by a set of nine parallel wires located on anarcuate plane, acting solely as charge amplifiers and equidistant fromthe two corresponding half-cathodes. The wires are brought to a highvoltage by a resistor 22'. The system of nine aligned cells isassociated with a matrix comprising n 3 lines and N 3 columns.

The half-cathodes 18', 18' are arranged in groups of three successiveinterconnected cathodes also connected to one of the N columns of amatrix network similar to that illustrated in FIG. 2. Clearly, the threesuccessive interconnected half-cathodes might also be united into asingle half-cathode shared by three cells.

Instead of disposing the two half-cathodes of the same cell on eitherside of the anode wire, they can be overlapped in the arrangementillustrated in FIG. 4 which shows a single cell: the two half-cathodes42, 44 overlapping one another at the same distance from the anode 46,so that the events of creating a signal are substantially identical forthe two half-cathodes of the same cell. One of the half-cathodes, forinstance 42, supplies a column, while the other half-cathode 44 suppliesa line. A signal processing and addressing device 48 similar to thatillustrated in FIG. 2 is connected to the lines and columns and suppliesthe counters 50 associated with each cell.

As stated hereinbefore, the invention enables the whole of an area or ameridian to be explored simultaneously, the result being a substantialreduction in experimental time in comparison with the conventionalsystems. The number of measuring channels for N X n cells is only N n:for a mosaic of 10,000 cells, there are 200 measuring channels, usingonly one square matrix. Although the detectors operate proportionally,the possible geometries are very various, since the filamentary anodeacts only as a charge amplifier.

In the foregoing description, the pulses corresponding to a nuclearevent localized in a cell of the detector are fed to a scaler associatedwith the cell; clearly, they can also be fed to a display system, forinstance, an oscilloscope, at the point defined by the coordinates ofthe cell, in the form of a light spot, and the light intensity thusproduced can be integrated either by using a long afterglow screen, orby photographing the screen with adequate exposure time.

What is claimed:

1. An ionizing detector system comprising an enclo sure of substantiallyuniform thickness, a fluid in said enclosure, a plurality N X n ofdetector cells within the enclosure, (where N is a predetermined integergreater than 1 and n is a predetermined integer greater than l) eachcell comprising an anode wire, a first flat halfcathode and a secondflat half-cathode located on the other side of the anode wire withrespect to the first half-cathode, a first group of N generally flatmetal strips providing the first half-cathodes of respective groups of ncells and a second group of n generally flat metal strips providing thesecond half-cathodes of respective groups of N cells, N X n countingmeans each arranged to count pulses from a respective one of the N X ndetector cells, N first communication paths each connecting a respectivestrip of the first group to n counting means through coincidence means,n second communication paths each connecting a respective strip of thesecond group to N counting means through said coincidence means, andmeans for applying between the half-cathodes and the anode of eachdetector cell a DC. potential difference such that each cell operates asa proportional counter.

2. A detector system according to claim 1, including inhibiting meansfor selecting one first path and one second path only in response toeach ionizing event, comprising, on each first or second path,discriminator means and blocking means, said discriminator meansdelivering a signal when a pulse exceeding a predetermined threshold isapplied thereto and said blocking means blocking transmission of pulsesfor a predetermined time period by all other first or second paths,respectively, in response to reception of said signal by thediscriminator means.

3. A detector system according to claim 1, each counting means having anaddress y l Ac, wherein l is an even number between 1 and N designatingthe row associated with the cell, c is an even number between 1 and ndesignating the column associated with the cell, and A is apredetermined integer greater than N and including coincidence meansdelivering the address y of the corresponding counting means in responseto any combination of row and column.

4. A system as set forth in claim 1 wherein the halfcathodes are broughtto do. earth potential, while the anode is brought to the dc. highvoltage.

5. A system as set forth in claim 1 wherein the two half-cathodes havesubstantially the same area and are disposed at the same distance fromthe filamentary anode.

6. A system as set forth in claim 1 wherein the anodes are formed by atleast one layer of parallel wires.

7. A system as set forth in claim 1 wherein the first half-cathodes ofthe cells are a group of metal strips parallel with a first directionand the second halfcathodes are a group of strips located on the otherside of the anodes and parallel to a second direction.

8. A system as set forth in claim 7 wherein the strips in one group areperpendicular to the strips in the other group.

1. An ionizing detector system comprising an enclosure of substantiallyuniform thickness, a fluid in said enclosure, a plurality N X n ofdetector cells within the enclosure, (where N is a predetermined integergreater than 1 and n is a predetermined integer greater than 1) eachcell comprising an anode wire, a first flat half-cathode and a secondflat half-cathode located on the other side of the anode wire withrespect to the first half-cathode, a first group of N generally flatmetal strips providing the first half-cathodes of respective groups of ncells and a second group of n generally flat metal strips providing thesecond half-cathodes of respective groups of N cells, N X n countingmeans each arranged to count pulses from a respective one of the N X ndetector cells, N first communication paths each connecting a respectivestrip of the first group to n counting means through coincidence means,n second communication paths each connecting a respective strip of thesecond group to N counting means through said coincidence means, andmeans for applying between the half-cathodes and the anode of eachdetector cell a D.C. potential difference such that each cell operatesas a proportional counter.
 1. An ionizing detector system comprising anenclosure of substantially uniform thickness, a fluid in said enclosure,a plurality N X n of detector cells within the enclosure, (where N is apredetermined integer greater than 1 and n is a predetermined integergreater than 1) each cell comprising an anode wire, a first flathalf-cathode and a second flat halfcathode located on the other side ofthe anode wire with respect to the first half-cathode, a first group ofN generally flat metal strips providing the first half-cathodes ofrespective groups of n cells and a second group of n generally flatmetal strips providing the second half-cathodes of respective groups ofN cells, N X n counting means each arranged to count pulses from arespective one of the N X n detector cells, N first communication pathseach connecting a respective strip of the first group to n countingmeans through coincidence means, n second communication paths eachconnecting a respective strip of the second group to N counting meansthrough said coincidence means, and means for applying between thehalf-cathodes and the anode of each detector cell a D.C. potentialdifference such that each cell operates as a proportional counter.
 2. Adetector system according to claim 1, including inhibiting means forselecting one first path and one second path only in response to eachionizing event, comprising, on each first or second path, discriminatormeans and blocking means, said discriminator means delivering a signalwhen a pulse exceeding a predetermined threshold is applied thereto andsaid blocking means blocking transmission of pulses for a predeterminedtime period by all other first or second paths, respectively, inresponse to reception of said signal by the discriminator means.
 3. Adetector system according to claim 1, each counting means having anaddress y 1 + lambda c, wherein 1 is an even number between 1 and Ndesignating the row associated with the cell, c is an even numberbetween 1 and n designating the column associated with the cell, andlambda is a predetermined integer greater than N and includingcoincidence means delivering The address y of the corresponding countingmeans in response to any combination of row and column.
 4. A system asset forth in claim 1 wherein the half-cathodes are brought to d.c. earthpotential, while the anode is brought to the d.c. high voltage.
 5. Asystem as set forth in claim 1 wherein the two half-cathodes havesubstantially the same area and are disposed at the same distance fromthe filamentary anode.
 6. A system as set forth in claim 1 wherein theanodes are formed by at least one layer of parallel wires.
 7. A systemas set forth in claim 1 wherein the first half-cathodes of the cells area group of metal strips parallel with a first direction and the secondhalf-cathodes are a group of strips located on the other side of theanodes and parallel to a second direction.